1. Field of the Invention
The present invention relates generally to apparatus for optically recording and reproducing information in record tracks on a rotatable record disc and, more particularly, to an improved apparatus for recording and reproducing information in cooperation with a rotatable record disc which is available for optically erasing or writing an information signal in record tracks provided thereon and for optically reading an information signal written in the record tracks provided thereon.
2. Description of the Prior Art
There has been proposed a so-called optical writable disc as one type of record disc which is available for optically writing an information signal in record tracks provided thereon and optically reading an information signal written in the record tracks provided thereon. A magnetooptic disc is one of such optical writable discs, is distinguished in that erasure of information signals and writing of information signals are able to be conducted repeatedly thereon. The magnetooptic disc comprises a disc shaped base, a vertically magnetized layer provided on a surface of the disc shaped base, and a protective layer formed to cover the vertically magnetized layer. Recording of information on the magnetooptic disc is achieved by erasing an information signal written thereon and writing a new information signal thereon, and reproduction of information from the magnetooptic disc is achieved by reading an information signal written thereon.
In the event of the erasure of information signals or the writing of information signals, the magnetooptic disc is rotated at a predetermined speed on a rotation axis at the center thereof and a laser light beam is caused to be incident upon the vertically magnetized layer of the magnetooptic disc on which a predetermined external magnetic field acts, so that portions of the vertically magnetized layer on which the laser light beam impinges are subject to an increase in temperature and provided with a direction of magnetization in accordance with the external magnetic field acting thereon. When an information signal is erased from the magnetooptic disc, the vertically magnetized layer of the magnetooptic disc is provided with a uniform direction of magnetization by the impingement of the laser light beam, and when an information signal is written on the magnetooptic disc, the vertically magnetized layer of the magnetooptic disc is provided with inversions in the direction of magnetization in a predetermined pattern as a result of the impingement of the laser light beam.
Further, in the event of the reading of information signals, another laser light beam having power smaller than that of the laser light beam used for the erasure or writing of information signals is caused to be incident upon the vertically magnetized layer of the magnetooptic disc which is rotated and a reflected laser light beam from the vertically magnetized layer of the magnetooptic disc is detected by a photosensitive device, so that a reproduced information signal corresponding to the information signal written in the magnetooptic disc is obtained from the photosensitive device. The reflected laser light beam obtained from the vertically magnetized layer of the magnetooptic disc in the case of the reading of information signals is provided, in accordance with the Kerr effect, with rotations of a polarization plane thereof in response to inversions in the direction of magnetization made in the vertically magnetized layer to represent the information signals written therein. Such rotations of the polarization plane of the reflected laser light beam are detected by the photosensitive device by means of detecting each of orthogonal polarized components of the reflected laser light beam separately and comparing detection outputs of the orthogonal polarized components with each other to produce a comparison output. Then, the reproduced information signal is obtained based on the comparison output.
In the magnetooptic disc on which the erasure, writing and reading of information signals are effected as mentioned above, a plurality of circular record tracks are formed coaxially in the vertically magnetized layer to surround a central portion thereof. Each record track is partitioned into a plurality of sectors and scanned by the laser light beam when the magnetooptic disc is rotated at the predetermined speed, so that the recording of information or the reproduction of information is carried out in units of one track or one sector.
Accordingly, a recording and reproducing apparatus for recording information on the magnetooptic disc and reproducing information from the magnetooptic disc comprises a disc driving portion for rotating the magnetooptic disc, a magnetic field generating portion for producing a predetermined magnetic field acting upon the magnetooptic disc, an optical head portion located to face the magnetooptic disc loaded on the disc driving portion for generating a laser light beam and causing the same to be incident upon the record track formed in the vertically magnetized layer of the magnetooptic disc, and a photodetecting portion for detecting a reflected laser light beam from the record track formed in the vertically magnetized layer of the magnetooptic disc to produce a reproduced information signal.
In a previously proposed apparatus for recording information on the magnetooptic disc and reproducing information from the magnetooptic disc, an optical head device for causing a single laser light beam to be incident upon the record track formed in the vertically magnetized layer of the magnetooptic disc is employed and the single laser light beam emitted from the optical head device is used for both the erasure or writing of information signals and the reading of information signals, selectively. In order to achieve such alternative use of the single laser light beam, a power control for the optical head device is carried out in such a manner that the laser light beam is emitted form the optical head device with relatively large power to be used for erasing information signals or writing information signals in the record track and with relatively small power to be used for reading information signals from the record track, and a switching control for the magnetic field generating portion is also carried out for causing the magnetic field produced by the magnetic field generating portion to have opposite directions for use in the writing or erasure of information signals and in the reading information signals, respectively.
However, in the previously proposed apparatus in which the single laser light beam emitted from the single optical head device is used for both the writing or erasure of information signals in the vertically magnetized layer of the magnetooptic disc and the reading of information signals from the vertically magnetized layer of the magnetooptic disc, selectively, there is a problem that a dead time is wasted to wait for completion of each turn of the magnetooptic disc when information signals are written in a selected position on the magnetooptic disc and therefore it is difficult to increase recording speed on the magnetooptic disc. For example, when an information signal written in a selected sector contained in a certain one of the record tracks which are formed in the vertically magnetized layer of the magnetooptic disc is erased and then a new information signal is written in the selected sector and thereafter read from the selected sector to be verified, first, the laser light beam emitted from the optical head device to be incident upon the vertically magnetized layer of the magnetooptic disc is provided with power for reading and subjected to a positional control so as to impinge upon the selected sector under a situation wherein address data recorded in the sectors in each record track are read by the laser light beam. Then, the laser light beam impinging upon the selected sector is changed to be provided with power for recording or erasing, so that the information signal written in the selected sector is erased. After that, the new information signal is written in the selected sector when one turn of the magnetooptic disc is completed after the start of the erasure of the information signal from the selected section. On the occasion of writing the new information signal the magnetic field procuded by the magnetic field generating portion is switched to have an inverted direction. Further, when another turn of the magnetooptic disc is completed after the start of the writing of the new information signal in the selected section, the laser light beam impinging upon the selected section is changed again to be provided with the power for reading and the new information signal which has been written in the selected sector is read to be verified by the laser light beam.
Accordingly, after the laser light beam emitted from the optical head device to be incident upon the vertically magnetized layer of the magnetooptic disc is caused to impinge upon the selected sector, three turns of the magnetooptic disc are required for erasure the information signal from the selected section, writing the new information signal in the selected section, and reading the new information signal from the selected section for verification, respectively, and in a period of each turn of the magnetooptic disc, a dead time is caused after the selected section is scanned by the laser light beam. Consequently, high speed recording of information signals on the magnetooptic disc is prevented from being realized.
Further, in the previously proposed apparatus mentioned above, it is required to control the magnetic field produced by the magnetic field generating portion to have opposite directions for use in the writing of information signals and in the reading of information signals, respectively. Accordingly, the magnetic field generating portion may be composed of a permanent magnet and a magnet inverting device for inverting the permanent magnet mechanically or an electromagnet and a magnet inverting circuit device for inverting the direction of an exciting current supplied to the electromagnet, and therefore, is undesirably complicated in construction. Besides, the inversion of the direction of the magnetic field by means of the magnet inverting device or the magnet inverting circuit device is not conducted at high speed usually, and this is a further impediment to the high speed recording of information signals on the magnetooptic disc.
When the writing of information signals is carried out in units of one sector on the magnetooptic disc by the apparatus for recording and reproducing information, it is usual that the information signal is written in each sector in the form of digital data. In such case, it is possible that the digital data written in the sector will contain errors which can not be corrected by error correction performed when the digital data are read from the magnetooptic disc and as a result proper reproduced information signals are not obtained. Accordingly, it has been proposed to provide alternate sectors which are distinct from sectors in which the digital data are normally written, that is, so-called user's sectors, at predetermined positions on the magnetooptic disc and used for rewriting of digital data which are written in the user's sector with errors unable to be corrected by the error correction.
In writing digital data in units of one sector on the magnetooptic disc which is provided with the alternate sectors, the digital data are written in the user's sector and then read from the user's sector for verification. In the case where the verification indicates that proper reproduced information signals can not be obtained from the digital data written in the user's sector, the same digital data are rewritten in the alternate sector and the digital data rewritten in the alternate sector is read in place of the digital data written in the user's sector on the occasion of the reproduction of information, so that alternation of sectors is effected.
However, when digital data are written in units of one sector in the magnetooptic disc provided with the alternate sectors by the apparatus aforementioned a relatively long time is required, after writing the digital data in the user's sector, for verifying the digital data written in the user's sector, causing the laser light beam to reach the alternate sector, and rewriting the digital data in the alternate sector, and accordingly data transfer speed is substantially reduced.